1. Field of the Invention
The present invention relates to an improved solar cell module which excels in weather resistance and exhibits a stable, desirable photoelectric conversion efficiency over a long period of time. More particularly, it relates to a solar cell module having an a-Si solar cell element provided with a grid electrode having a coating comprising a specific epoxy resin disposed so as to cover the entire exterior of the grid electrode.
2. Related Background Art
In recent years, heating of the earth because of the so-called greenhouse effect due to an increase of atmospheric CO.sub.2 has been predicted. Thus, there is an increased demand for a means of power generation capable of providing clean energy without causing CO.sub.2 buildup. In this regard, nuclear power generation has been considered advantageous in view of not causing CO.sub.2 buildup. However, nuclear power generation unavoidably produces radioactive wastes which are harmful for living things, and there is a probability that leakage of injurious radioactive materials from the nuclear power generation system will happen when the system is damaged. In this respect, early realization of a power generation system capable of providing clean energy without causing CO.sub.2 buildup as in the case of thermal power generation and without causing radioactive wastes and radioactive materials as in the case of nuclear power generation is an increased societal demand.
There have been various proposals to meet such societal demand. Among those proposals, solar cells are expected to be a future power generation source since they supply electric power without causing the problems mentioned above.
There have been proposed a variety of solar cells for commercial and home appliances. These solar cells include single crystal silicon cells, polycrystal silicon solar cells, and amorphous silicon solar cells.
As for single crystal silicon solar cells, there is a disadvantage that they are still costly because they use an expensive specific single crystal substrate. Hence, they have not yet come into general use as solar cells in commercial and home appliances used by the general public.
In view of this, public attention has been focused on polycrystal silicon solar cells and amorphous silicon solar cells. They can be relatively easily produced and are of low production cost, although they do not provide a photoelectric conversion efficiency as high as that provided by the single crystal silicon solar cells.
However, as for polycrystal silicon solar cells, there is a disadvantage in that it is difficult to form the semiconductor layer comprised of polycrystal silicon (hereinafter referred to as poly-Si) in a large area with a relatively low production cost.
On the other hand, amorphous silicon solar cells have been evaluated as being the most advantageous among the conventional solar cells because their semiconductor layer, comprised of amorphous silicon (hereinafter referred to as a-Si), can be easily formed in a large area and in a desired form on a relatively inexpensive substrate of glass, metal, or synthetic resin with a relatively low production cost.
As for amorphous silicon solar cells (hereinafter referred to as a-Si solar cells), the production cost has been estimated to be markedly lower than that of single crystal silicon solar cells when the production reaches several hundreds of megawatts. In view of this, various studies have been made on a-Si solar cells from various viewpoints such as reproducibility, productivity, production cost, etc.
Now, an a-Si solar cell module capable of outputting an electric power of several watts or above is usually used outdoors and because of this, it is required to have relevant weathering resistances in terms of resistance to rain (water), dust, ultraviolet rays, heat, humidity, and the like. In the production of an a-Si solar cell module, an a-Si solar cell is encapsulated using a glass plate or a synthetic resin so that the above requirements are fulfilled.
In the case where the a-Si solar cell is encapsulated using a glass plate, it is necessary for the glass plate to be of a thickness of several millimeters in order to prevent the glass plate from being broken. In this case, the a-Si solar cell module unavoidably becomes relatively heavy and costly. In addition, the a-Si solar cell module is poor in flexibility.
In the case of producing a solar cell module using an a-Si solar cell element with a bendable substrate such as stainless steel foil, synthetic resin film, or the like, the a-Si solar cell is encapsulated using a synthetic resin member such as a film of fluorine-containing resin, ethylene-vinyl acetate copolymer (EVA), or the like so that the resulting a-Si solar cell module becomes bendable as desired.
In the following, explanation will be made of the conventional a-Si solar cell module shown in FIG. 2.
FIG. 2 is a schematic view illustrating the configuration of a conventional a-Si solar cell module. In FIG. 2, reference numeral 200 indicates an a-Si solar cell element which comprises a metal electrode layer 202, an amorphous silicon semiconductor layer (hereinafter referred to as a-Si semiconductor layer) 203 having a pin junction, and a transparent and conductive layer 204 disposed in this order on a conductive substrate 201. Reference numeral 205 indicates a grid electrode. Reference numeral 206 designates a filler provided so as to enclose the entire exterior of the a-Si solar cell element 200. Reference numeral 207 indicates a weather-resistant protective member (or a surface protective member) made of a synthetic resin which is disposed on the surface of the filler 206. Reference numeral 208 indicates a base member (or rear face protective member).
The grid electrode 205 is usually formed using a paste composed of powdered Ag and binder resin in order to provide it with a desirable flexibility and a desirable resistance to changes in temperature. As the binder resin used in this case, there are known polyester resins exhibiting good flexibility, epoxy resins exhibiting good weather resistance, and other than these, phenol resins.
In order to obtain an a-Si solar cell module of the configuration shown in FIG. 2 which excels in flexibility, the grid electrode 205 is formed using Ag paste comprising polyester resin as the binder dispersed in powdered Ag.
The weather-resistant protective member 207 is usually formed of a fluorine-containing resin so that it exhibits water resistance, resistance to dust, weather resistance, etc. required therefor.
As for the filler 206 which encloses the solar cell element 200, it is required to be transparent and highly insulative and to have weather resistance, water resistance, and high-impact properties. Besides these requirements, it is also required to exhibit a good adhesion property to the solar cell element 200 and not to have any negative influence on other constituent members. In view of this, as the filler 206, there have been used EVA, silicone resin, or polyvinyl butyral (PVB). Among these resins, EVA is most generally used since it has various advantages such that it can be handled in a sheet-like state, it can be easily processed, and it is relatively inexpensive.
Now, there is a problem with the above-described conventional a-Si solar cell module such that not only the weather-resistant protective member 207 formed of a fluorine-containing resin but also the filler 206 comprised of EVA has a tendency to allow moisture to pass through them, although they have appropriate weather resistance and water resistance.
Besides this problem, there is another problem with the above-described conventional solar cell module such that the grid electrode 205 formed of the foregoing Ag paste comprising powdered Ag and polyester resin binder is apt to allow moisture to pass through the grid electrode. The moisture eventually arrives at the portion situated under the grid electrode because the Ag paste contains voids, although it meets the requirement relative to the flexibility needed therefor. This situation often occurs when the above-described conventional solar cell module is irradiated with light (sunlight) under conditions of high temperature and high humidity, wherein moisture penetrates the filler 206 and the grid electrode 205 and ultimately reaches the a-Si semiconductor layer 203 through the transparent and conductive layer 204. When the a-Si semiconductor layer 203 is accompanied by defective portions such as pinholes, the moisture passes through those defective portions of the a-Si semiconductor layer and reaches the metal electrode layer 202 (that is, the lower electrode), causing the grid electrode 205 to be short-circuited with the metal electrode 202 and thus reducing the solar cell characteristics.
In order to confirm the above problematic situation with respect to the conventional a-Si solar cell module in which the grid electrode is formed of the foregoing Ag paste comprising powdered Ag and polyester resin binder, a voltage was externally applied between the grid electrode and the metal electrode layer (that is, the lower electrode) so that a positive bias voltage was applied to the grid electrode. As a result, the above problems relative to the occurrence of short-circuits between the grid electrode and the lower electrode and to the reduction in photoelectric conversion efficiency were observed.
Occurrence of the above problems in the conventional a-Si solar cell module is considered to be due to the following reasons. That is, in the case where moisture has penetrated into the a-Si solar cell element 200 as above described upon subjecting the a-Si solar cell element to irradiation of light (sunlight) under condition of high temperature and high humidity, a positive voltage is applied to the grid electrode 205 because of the penetrated moisture and a self-generated photoelectromotive force of the a-Si solar cell element causes electrochemical reaction of the Ag contained in the grid electrode, whereby the Ag is oxidized in the moist media to provide Ag.sup.+ ions. When the Ag.sup.+ ions thus generated are diffused into the a-Si semiconductor layer, a remarkable reduction is caused in the photoelectric conversion efficiency of the a-Si solar cell element. And in the case where the a-Si semiconductor layer has defect portions such as pinholes, the Ag.sup.+ ions reach the lower electrode (the metal electrode) through these defect portions, and they are reduced to metallic Ag. Should this reaction happen continuously, the metallic Ag eventually grows to produce a dendritic crystal, resulting in electrical connection of the grid electrode to the lower electrode, thereby causing short-circuiting of the a-Si solar cell element. As a result, a marked reduction is caused in the photoelectric conversion efficiency of the a-Si solar cell element.
The detailed reaction mechanism in this case is not clear but it is considered that the following reactions are likely to be caused in the a-Si solar cell element:
(1) at the grid electrode (which serves as an anode): EQU Ag.sub.2 O+H.sub.2 O- - - 2Ag.sup.+ +20H.sup.-, and
(2) at the lower electrode (which serves as a cathode): EQU Ag.sup.+ - - - e.sup.- - - - Ag(precipitation of dendritic crystal).
In order to prevent occurrence of the above-described short-circuit between the grid electrode and the lower electrode, there is a proposal to use epoxy resin instead of the polyester resin as the binder resin of the Ag paste for the formation of the grid electrode. However, as for an a-Si solar cell of the configuration shown in FIG. 2 in which the grid electrode 205 is formed of a paste comprising powdered Ag and epoxy resin, because the epoxy resin is hard and is insufficient in flexibility, although its moisture permeability is low, the grid electrode is likely to separate from the a-Si solar cell element body upon exposure to changes in temperature in repeated use, wherein the contact resistance between the grid electrode and the a-Si solar cell element body is raised, causing a marked reduction in the photoelectric conversion efficiency. Thus, the use of epoxy resin instead of the polyester resin as the binder resin of the Ag paste is not effective to solve the above problem.
Besides the above, in order to prevent occurrence of the above-described short-circuit between the grid electrode and the lower electrode, it is considered effective to replace EVA as the filler 206 with another resin having a lower moisture permeability.
In the technical field to which the present invention pertains, the degree of moisture which permeates a given material is evaluated by observing the so-called moisture permeability in accordance with the method stipulated in JIS-Z0208, in which how much moisture permeates said given material with a definite thickness and a definite area within a prescribed period of time under a specified temperature and moisture is measured.
The moisture permeability of EVA (ethylene-vinyl acetate copolymer) is 20 to 200 (g/m.sup.2 .multidot.day.multidot.0.1 mm/40.degree. C..multidot.90%RH.
In view of this, other resins having a moisture permeability which is lower than that of EVA are resins having a moisture permeability of less than the above-mentioned moisture permeability value of EVA, that is, 20 g/m.sup.2 .multidot.day.multidot.0.1 mm/40.degree. C..multidot.90%RH. Specific examples of such resins are polyethylene, polypropylene, polyvinyl alcohol, polymethyl methacrylate, polyethylene terephthalate, vinylidene chloride-vinyl chloride copolymer, and epoxy resin.
However, of these resins, polyethylene, polypropylene, polyvinyl alcohol, polymethyl methacrylate, polyethylene terephthalate, and vinylidene chloride-vinyl chloride copolymer are not usable instead of EVA as the filler 206 because each of them is poor in adhesion with the exterior of the solar cell element 200. Particularly, when an a-Si solar cell module of the configuration shown in FIG. 2 in which the filler 206 is comprised of a member selected from polyethylene, polypropylene, polyvinyl alcohol, polymethyl methacrylate, polyethylene terephthalate, and vinylidene chloride-vinyl chloride copolymer is bent, the filler 206 is likely to separate from the solar cell element. In addition, when said solar cell module is repeatedly exposed to high temperature, the filler 206 situated in the vicinity of the interface with the exterior of the solar cell element 200 is colored white and eventually separates from the exterior of the solar cell element. Further, when said solar cell is placed outdoors so as to be exposed to sunlight over a long period of time, the filler 206 is yellowed.
In the case of an a-Si solar cell module of the configuration shown in FIG. 2 in which the filler 206 is comprised of epoxy resin, there also is a problem that the filler is yellowed when the solar cell module is placed outdoors so as to be exposed to sunlight over a long period of time, resulting in a remarkable reduction in the photoelectric conversion efficiency of the solar cell element. This situation is apparent from FIG. 3, which shows a graph of the changes in light transmittance of EVA and epoxy resin when exposed to sunlight. Thus, it is apparent that epoxy resin is also not usable in place of EVA as the filler 206.